1. Field of the Invention
The present invention relates to polymer material used for composing optical communication parts, optical waveguides using such polymer materials and a method of fabricating the optical waveguides.
2. Description of the Background Art
In the field of telecommunications, development of optical waveguide has been recognized as a critical issue to enable large capacity communications.
Prerequisites to the materials used for composing optical communication parts such as an optical waveguide include higher transparency at wavelengths in the near-infrared range to which the wavelength of optical signals belongs and less scattering. The materials are also required to have controllability in their refractive indices since they are used to compose optical transmission paths.
Glass or other inorganic crystalline materials have conventionally been used as materials for composing optical communication parts such as an optical waveguide. These materials, however, suffer from their expensiveness and difficulty in processing.
In recent years, polymer materials, such as PMMA (polymethyl methacrylate) and PS (polystyrene), became more popular thanks to their inexpensiveness and easier processing as compared with those of glass or other inorganic crystalline materials. Use of such material can provide a film-type optical waveguide with wider area and higher flexibility than the conventionals. It becomes also possible to obtain a functional optical waveguide by introducing functional compounds or functional groups into such polymer materials.
Fabricating such an optical waveguide essentially requires a method of processing the polymer materials into a desired form. Typical of such a method has been the reactive ion etching (RIE) method using oxygen plasma. The fabrication process of a polymer-made optical waveguide by the RIE method has to be proceeded as generally shown in FIGS. 5A to 5E. Here, FIGS. 5A to 5E show schematic cross-sectional views useful for understanding the major steps in sequence for fabricating a polymer-made optical waveguide using the RIE method.
First, on a base 101, a polymer film 103a as an underclad, a polymer film 103b for forming a core, and a photoresist film 105 for forming an etching mask are formed in this order, FIG. 5A.
To obtain the etching mask corresponded to a desired patterned shape by processing the photoresist film 105, the photoresist film 105 is then subjected to selective light exposure through a photomask 107, FIG. 5B, corresponding to the patterned shape. This results in forming a latent image of the pattern in the photoresist film 105. The photoresist film 105 after exposed with the light is then developed to obtain a resist pattern 105x, FIG. 5C. The example shown here relates to a case with negative photoresist.
RIE with an oxygen-base etching gas is then carried out using the resist pattern 105x as an etchingmask 105x, and aportion of the polymer film 103b being exposed from the etching mask 105x is removed. A core 103x made of the residual portion of the polymer film 103b is thus formed on the underclad 103a, FIG. 5D.
On the specimen on which the core 103x has been formed, a polymer film 111 for forming overclad is formed to obtain an optical waveguide 113, FIG. 5E. The overclad 111 can be formed by, for example, coating on the specimen a coating fluid containing material of the overcdad, and is then allowed to dry.
As for PMMA and PS, some approaches have been taken to improve transparency in the near-infrared region. More specifically, these materials show absorption ascribable to C--H bonds in their molecules in the near-infrared region, and thus deuterated PMMA, that is, PMMA whose hydrogen atoms are substituted with deuterium atoms has been developed. Deuterated PMMA shows absorption in the far-infrared region as shifted from the near-infrared region.
The above-described PMMA, PS and deuterated PMMA composing the core of the optical waveguide, however, are low in glass transition temperature. For instance, both of the PMMA and deuterated PMMA have a glass transition temperature of 107.degree. C., so that these materials may easily be softened due to heat treatment such as soldering, if they are used to compose electronic parts for computers or so.
These materials also suffer from relatively high water absorption. Both of the PMMA and deuterated PMMA have a value of water absorption as high as 2.0%. The materials composing optical communication parts may alter their refractive indices due to water absorption, which may cause undesirable transmission error in optical communications.
The PS further has a specific problem on birefringence. In the conventional fabrication process of optical waveguides based on the RIE method, a number of steps and a long process time are necessary for forming the pattern, as is clear from the description referring to FIGS. 5A to 5E. Problems also reside in that apparatus used for the RIE method costs high and requires special skills in the operation.